1. Field of the Invention
The present invention relates to a semiconductor memory in which a word line has a shunt and to a method of fabricating the same.
2. Description of the Prior Art
A word line of a semiconductor memory, i.e., a gate line of an access transistor, is generally formed by a polysilicon film doped with an impurity or a polycide film. However, such a word line has a relatively high resistance. For this reason, a method of forming a shunt made of an Al film or the like to the word line is used to decrease the delay of the word line.
FIG. 1 schematically shows a DRAM using the above method. In this DRAM, each memory cell is constituted by an access transistor 11 and a capacitor 12. One word line formed by a polycide film 13 or the like is connected to one column of a memory cell array, and a shunt formed by an Al film 14 or the like is connected to the polycide film 13 every predetermined connecting region 15.
One end of the polycide film 13 is connected to one diffused layer of a transistor (not shown) in a column decoder 16. A bit line formed by a polycide film 17 or the like is connected to one line of the memory cell array.
FIGS. 2A and 2B show a conventional DRAM having the arrangement shown in FIG. 1. In this conventional DRAM, the polycide film 13 extends on an element isolation SiO.sub.2 film 21 in the connecting region 15, and a contact hole 23 is formed in an interlayer insulator 22 covering this polycide film 13.
The polycide film 17 formed by the same layer as that of the bit line and patterned into a square is brought into contact with the polycide film 13 through the contact hole 23. The polycide film 17 is covered with a BPSG film 24, and the Al film 14 is in contact with the polycide film 17 through a contact hole 25 formed in the BPSG film 24.
As shown in FIG. 3, according to another prior art, an Al film 14 may be in direct contact with a polycide film 13 without going through a polycide film 17.
Incidentally, charges are accumulated during a wafer process in the polycide film 13 which serves as the word line or Al film 14 which serves as the shunt of the word line. However, as described above, an end of the polycide film 13 is connected to only the diffused layer of the transistor in the column decoder 16.
The area of the diffused layer of the transistor in the column decoder 16 cannot be increased due to a limitation in a layout. Therefore, a resistance between the diffused layer and the semiconductor substrate is high, and charges accumulated in the polycide film 13 or the like are not easily discharged to the semiconductor substrate through the diffused layer.
For this reason, a large electric field is applied to the gate insulating film between the polycide film 13 and the semiconductor substrate, and carriers are injected from the semiconductor substrate and the diffused layer into the gate insulating film. As a result, the characteristics such as a threshold voltage of the transistor 11 are varied, or the gate insulating film is degraded and broken down. Therefore, according to the prior arts shown in FIGS. 2A, 2B, and 3, a highly reliable semiconductor memory is not always obtained.